Foams, chlorofluorocarbons

Aliphatic chlorocarbons and chlorofluorocarbons have found widespread commercial use as refrigerants, industrial solvents, and blow agents for the preparation of rigid polyurethane foams. Chlorofluorocarbons in the stratosphere are enviromnentaUy harmful because the photolytic cleavage of their carbon-chlorine bonds results in depletion of the... 

Chlorofluorocarbon-blown foam blocks are used to insulate the walls and roofs of some buildings, thus reducing heat losses and helping to conserve fossil fuels. In this area, polyurethane foam competes with polystyrene foam, which until recently was blown with dichlorodifluoromethane (CFC 12) but is now blown with a mixture of chlorodifluoromethane (HCFC 22) and 1 -chloro-l,l-difluoroethane (HCFC 142b). 

The term CFCs is a general abbreviation for ChloroFluoroCarbons. They have been extensively used since their discovery in the thirties, mainly as refrigerant, foam blowing agent, or solvent because of their unique properties (non toxic, non flammable, cheap). However, after the first warning of Rowland and Molina [1] in 1974 that CFCs could destroy the protective ozone layer, the world has moved rapidly towards a phase-out of CFCs. Because the destruction of stratospheric ozone would lead to an increase of harmful UV-B radiation reaching the earth s surface, the production and use of CFCs is prohibited (since January 1, 1995 in the European Union and since January 1, 1996 worldwide). 

In 1974, F. Sherwood Rowland and Mario Molina, who shared the 1995 Nobel Prize in Chemistry with Crutzen, showed that chlorine from photolyzed chlorofluorocarbons (CFCs) such as CF2C12 and CFCI3, which were used as supposedly inert refrigerants, solvents for cleaning electronic components, plastic foam blowing agents, and aerosol spray propellants, can also catalyze ozone loss. Subsequently, the chlorine monoxide molecule CIO, which is involved in the chlorine-catalyzed ozone destruction cycle, has been shown to be present in the holes in the ozone layer and to correlate inversely with... 

Chlorine can be substituted by hydrogen fluoride and boron trifluoride to yield chlorofluorocarbon compounds. This process is applied for producing 1,1-dichloro-l-fluoroethane (14), which is considered as a replacement for trichlorofluoromethane as a blowing agent for plastic foam.18... 

Chlorofluorocarbon Alternatives. There still is no completely satisfactory propellant for use in the aerosol method of foam production. Chloroflu-orocarbons, still widely used, are harmful 10 atmospheric ozone and low molecular weight hydrocarbons, now popular, e g., in producing shaving cream, are explosive and promote the greenhouse effect. See also Fluorine. 

Since the pores in an aerogel are comparable to, or smaller than, the mean free path of molecules at ambient conditions (about 70 nm), gaseous conduction of heat within them is inefficient. Coupled with the fact that solid conduction is suppressed due to the low density, a silica aerogel has a typical thermal conductivity of 0.015 W/ (m-K) without evacuation. This value is at least an order of magnitude lower than that of ordinary glass and considerably lower than that of CFC (chlorofluorocarbon)-blown polyurethane foams (54). 

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